Embodiments of the present invention relate to a transmitter. Further embodiments of the present invention relate to a transceiver. Some embodiments of the present invention relate to an electrically controllable directional antenna. Some embodiments of the present invention relate to a switched beam power combiner.
FIG. 1 shows a block diagram of a plurality of radio stations that are communicating by means of high frequency signals. Thereby, in FIG. 1 four radio stations 101 to 104 are shown by way of example. Each radio link (or radio communication) 121 to 125 is operated in both directions, i.e. bidirectional. Moreover, each radio station 101 to 104 comprises a combined transmit receive unit (transceiver) having a downstream signal processing (unit) 141 to 144 and a common antenna 161 to 164 for transmitting and receiving (or two separate antennas for transmitting and receiving). Each of the antennas 161 to 164 can comprise a plurality of interconnected or combined radiating elements. Furthermore, each radio station 101 to 104 is configured to set-up simultaneous radio links to one or more adjacent radio stations, or even to all radio stations 101 to 104, e.g. as shown in FIG. 2. Thereby, the radio links 121 to 125 form a network structure with the individual radio stations 101 to 104 as nodes. Note that in FIG. 2, a three dimensional Cartesian coordinate system is shown for illustration purposes. The Cartesian coordinate system comprises a x-axis, a y-axis and a z-axis, wherein the x-axis and the y-axis span a horizontal plane perpendicular to an earth gravitational vector, and wherein the z-axis (altitude) is parallel to the earth gravitational vector. Since differences in altitude (z-axis) between the individual radio stations 101 to 104 are small compared to distances between the radio stations 101 to 104, the spatial position of the individual radio stations 101 to 104 is primarily characterized by their horizontal angle or azimuthal angle (angle in the horizontal plane, e.g. relative to the x-axis or y-axis) 201 and 202.
FIG. 3 shows a block diagram of the radio stations 101 to 104 shown in FIG. 1, wherein two of the four radio stations 101 to 104 are configured as transmitters 101 and 104 and the other two as receivers 102 and 103. In order to increase the operating range (coverage) for a given transmit power, or even to implement a secure radio link 121, 123 or 125, the high frequency signals can be bundled by the antennas 161 and 164 of the transmitters 101 and 104 and be radiated (or transmitted) mainly in the direction of the receivers 102 and 103. Bundling of the beams can be performed by appropriate beam patterns 301 to 303 with distinctive main lobes. Amplification of the radio signals can be effected additionally or alternatively by beam patterns 304 to 306 on the receiver side. If the direct line of sight is blocked, e.g. by an obstacle 32, then a directed radio link 123 can be set-up by means of an appropriate reflector 34. In FIG. 3 only unidirectional radio links 121, 123 and 125 are shown by way of example, wherein an extension to bidirectional radio links can be performed by means of exchanging transmitters 101 and 104 and receivers 102 and 103. Hence, by means of forming (or generating) appropriate beam patterns, each transmitter is capable of radiating the transmit power simultaneously in one or more azimuthal directions and each receiver is capable of receiving signals from one or more azimuthal directions. Generally, the positions of the radio stations 101 to 104 are not fixed but variable, such that the alignment of the beams or main lobes has to be carried out dynamically.
Moreover, for a secure operation, the radio system necessitates that the transmit signals have a particular minimal power. The generation of the necessitated transmit power is particularly challenging for micro or millimeter wave radio systems.
Typical applications for the radio system shown in FIG. 1 are meshed outdoor networks, e.g. for a radio communication between vehicles, buildings or radio masts.
The simultaneous and adaptive formation of multiple main lobes is so far possible with MIMO signal processing (MIMO=Multiple Input Multiple Output). In these systems each antenna element (or radiating element) is connected via a separate transceiver and a separate digital-to-analog converter or analog-to-digital converter to a common digital signal processing (unit). The signal processing (unit) performs for each desired main lobe a separate complex weighting of the antenna signals, e.g. as described in D. Gesbert, M. Shafi, S. Da-shan, P. Smith and A. Naguib, “From theory to practice: an overview of MIMO space-time coded wireless systems”, Selected Areas in Communications, IEEE Journal on, vol. 21, no. 3, pp. 281-302, April 2003. The MIMO signal processing allows any manipulation of the transmit and receive signals not only for the entire signal but also for individual signal components in the time or frequency domain. Hence, the MIMO signal processing is very flexible and efficient. Thereby, all antenna elements contribute to the formation of the main lobes, wherein each antenna element usually comprises a low directivity. This solution offers the greatest flexibility at the highest effort because a transceiver including a converter is necessitated for each antenna element (or radiating element). The antenna array is usually planar and hence covers only an azimuthal angle of 120°. In principle, a curved or circular arrangement of the antenna elements (or radiating elements) is also possible in order to achieve a 360° covering. In this case, not all antenna elements can contribute to the formation of a specific main lobe, which results in a poor exploitation of the already complex system.
Besides, there is the possibility of using antenna arrays having switchable fixed main lobes (switched beam antennas). In this case, the antenna elements are connected via a change-over switch to the transceiver, such that at all times only one specified antenna element having a corresponding main lobe is active. The desired high directivity is realized within each antenna element (or radiating element). With this easy approach a simultaneous formation of main lobes is not possible. A more complex switched beam antenna can be realized by means of a Butler matrix or Rotman lens, e.g. as described in W. Rotman and R.F. Turner, “Wide-Angle Microwave Lens for Line Source Applications,” IEEE Transactions on Antennas and Propagation, November 1963. Thereby, the main lobes are formed by all antenna elements (or radiating elements) together, wherein the Butler matrix or Rotman lens comprises a separate input for each main lobe. Thus, the single antenna element has only a low directivity. If the transceiver is switched to one or more inputs, then one or more simultaneous main lobes can be formed. Switched beam antennas are predominantly assembled with planar antenna arrays with a consequently limited angular range in an azimuthal direction. In principle, a curved or circular arrangement is conceivable, but in this case as well not all antenna elements contribute to the formation of the main lobes resulting in the same disadvantages as in the antenna arrays with MIMO signal processing. Planar switched beam antennas with a Rotman lens are mainly applied in simple radar systems, e.g. for vehicles.
With mechanically rotating antennas the desired directivity is achieved by a single antenna element (or radiating element) that is rotated or tilted in the desired azimuthal direction. Advantageous is the perfect angular coverage of 360° and the relatively low technical effort, e.g. only one transceiver is necessitated, while the mechanical inertia and the fact that it is not possible to form more than one main lobe at the same time is disadvantageous. Mechanically rotatable antennas are primarily used for ship or plane radars.
Moreover, when using phased array antennas, the signals of individual antenna elements (or radiating elements) are weighted with complex factors (e.g. amplitude and phase), e.g. as described L. C. Godara, “Application of Antenna Arrays to Mobile Communications, Part II: Beam-Forming and Direction-of-Arrival Considerations,” Proceedings of the IEEE, vol. 85, no. 8, August 1997. Hence, each transmit antenna receives an identical, but individually weighted, signal. On the receiver side the individual antenna signals are individually weighted and added. In contrast to antenna arrays with MIMO signal processing, a complete transceiver for each antenna element (or radiating element) is not necessarily necessitated, e.g. a realization with an analog phase shifter for each antenna element may be sufficient. In this case a common transceiver can be used. An upstream or downstream signal processing (unit) on the transmitter or receiveer side thus has no influence on the individual antenna signals. With phased array antennas all antenna elements contribute to the formation of the main lobes. Thus, the single antenna elements only necessitate a small directivity. A planar arrangement of the antennas comprises a restricted angular coverage, wherein curved or circular arrangements have the above mentioned disadvantages. A simultaneous formation of multiple main lobes is limited and has a worse performance than with MIMO signal processing. Therefore, usually no use is made of this possibility. Phased array antennas are primarily used in radar systems and satellite communication, especially for replacing mechanically rotatable antennas.
There are several technical solutions for estimating the direction that differ in complexity of the transmit receive unit (transceiver) and the usable propagation conditions.
Systems with MIMO signal processing comprise the highest complexity because each transmit and receive antenna necessitates a separate transmit receive module. There are numerous known methods for estimating the direction with MIMO signal processing that are applied to mobile communication, e.g. the MUSIC (multiple signal classification) or ESPRIT (estimation of signal parameters via rotational invariance techniques) methods described in R. O. Schmidt, “Multiple Emitter Location and Signal Parameter Estimation,” IEEE Transactions on Antennas and Propagation, vols. AP-34, no. 3, March 1986.
In systems with beamforming signal processing only one transceiver is necessitated per radio station. Numerous methods are known for direction estimation with beamforming signal processing, e.g. as described in P.-J. Chung and J. F. Böhme, “Recursive EM and SAGE-Inspired Algorithms With Application to DOA Estimation,” IEEE Transactions on Signal Processing, vol. 53, no. 8, August 2005, D. J. Love and R. W. Heath, “Equal Gain Transmission in Multiple-Input-Multiple-Output Wireless Systems,” IEEE Transactions on Communications, Vol. 52, no. 7, July 2003, and S.-H. Wu, L.-K. Lin and S.-J. Chung, “Planar arrays hybrid beamforming for SDMA in millimeter wave applications,” Personal, Indoor and Mobile Radio Communications, PIMRC 2008, IEEE 19th International Symposium on, pp. 1-6, 2008. These methods are used, for example, for inter-communication using millimeter waves, e.g. as described in ECMA International, Standard ECMA-387: High Rate 60 GHz PHY, MAC and HDMI PAL, Geneva, 2008. These known approaches achieve their greatest performance in multi-path propagation channels. In the communication scenario shown in FIG. 1, an undisturbed line of sight or a single reflection (FIG. 3) of the main propagation path is assumed.
Besides, there is still the possibility to directly evaluate location information in order to calculate the spatial direction (azimuthal directions) of the transmit and receive antennas when the position of adjacent transceivers is known. Thereby, the location information can be obtained by means of satellite navigation, e.g. GPS (global positioning system). This solution necessitates that each transceiver is capable of performing a location determination and that each transceiver is capable of communicating this information to the other nodes of the network. Hence, each transceiver has to comprise a receiver for the satellite navigation and there has to be the possibility for a non-directional wireless data communication, such that the location information can be exchanged before the main lobes are formed. So far this method is used for dynamic alignment of phased array antennas towards airplanes for communicating with satellites.
In many application cases it is not possible or technically not feasible to generate high transmit powers in an integrated semiconductor. A further parallelization on the semiconductor device would result in large area consumption and high costs and is therefore often not feasible. Furthermore, a high performance cooling concept would be necessitated in order to dissipate the concentrated dissipation power.
For the generation of the necessitated power usually multiple power amplifiers are built up discretely and their outputs are combined. Previous solutions to this problem can be separated according to how the output signals of the power amplifiers are combined. When using mechanically oriented directional antennas as well as mechanically oriented directional antennas the power is merged in a power combiner. Power combiners usually comprise cascaded Wilkinson power combiners or 90°-hybrids. These can combine the signals of two inputs to an impedance-controlled output. For example, if 2n amplifiers should be parallelized, then this can be done by means of a binary tree structure comprising n levels. Depending on the number of amplifiers, these structures can adopt a large structural shape. Furthermore, losses of the power combiner increase with an increasing number of amplifiers, such that from a certain number of amplifiers no significant increase of output power can be achieved. Moreover, inequalities of individual amplifiers may also lead to losses.
When using MIMO signal processing with separate transceivers per antenna, the signals of the amplifiers are combined in the air as electromagnetic waves, such that the power combiner as a dedicated component can be omitted.
More complex systems with beamforming signal processing comprise separate transceivers per antenna element (or radiating element) as well. The complex weighting is thereby integrated in the transceivers. Separation or combination of the weighted antenna signals can be performed directly downstream the digital-to-analog conversion or directly before the upstream analog-to-digital conversion. The combination of the transmit signals of the individual transmit amplifiers takes place in the air as electromagnetic waves as well.